In Long Term Evolution-Advanced (LTE-A), which is the next-generation cellular communication standard discussed in Third Generation Partnership Project (3GPP), introduction of technology called carrier aggregation (CA) has been studied. The carrier aggregation is technology that forms a communication channel between user equipment (UE) and a base station (BS, or evolved Node B (eNB)) by aggregating a plurality of frequency bands that are supported in LTE, for example, and thereby improves communication throughput. Each frequency band included in one communication channel by the carrier aggregation is called a component carrier (CC). The bandwidths of frequency bands that are available in LTE are 1.4 MHz, 3.0 MHz, 5.0 MHz, 10 MHz, 15 MHz, and 20 MHz. Accordingly, if five bands of 20 MHz are aggregated as component carriers, a communication channel of 100 MHz in total can be formed.
In LTE-A HetNet scenarios, e.g., a number of small cells, such as micro cell, pico cells and/or femto cells are deployed on top of the regular macro cells, the same carrier frequency is applied for sake of spectrum efficiency. As the inter-cell interferences in-between macro cells and small cells turn out to be severe and have different characters from the traditional pure macro cell scenarios, to reduce strong uplink interference from macro cells to small cells, one positive bias (generally the value is 6 dB or 9 dB) is added into the handover triggering condition when evaluating the downlink signal quality of small cells. This is equivalent to expanding the cell range of the small cell when a UE takes handover between the macro cell and the small cell. This technique is called cell range extension (CRE). More information about CRE can be found in 3GPP TS36.300, E-UTRA Overall Description, such as V11.4.0, December, 2012. In this way, for a UE in the macro cell but close to cell border of the small cell, its distance to the base station of the small cell is increased, so the uplink interference to the small cell caused by the UE is reduced. However, the UEs who lie in the extended area in the small cell, would suffer from strong downlink interference from the macro cell.
When more frequency spectrums are exploited, in order to allow maximum single-UE peak throughput, a number of overlapping cells are deployed by both macro-eNB and small-eNB (such as micro-eNB, pico-eNB or femto-eNB), one cell at one frequency carrier, as shown in FIG. 1. Cell 1 and cell 2 are deployed by one macro-eNB, with cell 1 over frequency 1 and cell 2 over frequency 2 respectively, while cell 3 and cell 4 are deployed by one small-eNB, with cell 3 over frequency 1 and cell 4 over frequency 2 respectively.
This kind of network is called as “HetNet with multiple frequency carriers”. Carrier aggregation can be enabled in these co-deployed inter-frequency cells. To reduce the interference on control channel from macro cells to small cells, the technique of cross-carrier scheduling (CCS) which is extended from carrier aggregation could be exploited. Its idea is that different carrier frequencies are allocated to a certain macro cell (cell 2) and a certain small cell (cell 3) respectively as dedicated primary cells (PCells), and the downlink control channels for secondary cells (SCells) (cell 1 and cell 4) are transmitted in these dedicated primary PCells with explicit carrier indicator. In this way, the inter-cell interference on PCell control channel can be avoided. For small cell UEs who lie in an CRE area (we call it CRE-area): downlink control channels of the dedicated PCell is protected from interference of neighbor cells, which dedicated PCell is called protected cell in general; while the dedicated SCell is not protected for downlink control channels, which is called non-protected cell in general. It should be noted that the technique of CCS aims to provide protection only to downlink control channels to the small cell. More information about CCS can be found in 3GPP TS36.212, E-UTRA Multiplexing and channel coding, such as V11.3.0, June, 2013.
Then, for the geographical areas served by the small cells, one portion (inside the original cell border, called non-CRE area) suffers from weak or negligible interference in downlink control channels in any frequency carriers, where downlink control information transfer is available at both the protected cells and the non-protected cells; and the other portion (inside the extended cell border and outside the original cell border, called CRE area) suffers from strong or dominant interference in downlink control channels in the frequency carrier of PCell of overleaped macro cell, where downlink control information transfer is available at only the protected cells rather than non-protected cells. So, over frequency 1, non-CRE area is of “low-interference” area, and CRE area is of “high-interference, protected” area; over frequency 2, non-CRE area is of “low-interference” area, and CRE area is of “high-interference, non-protected” area.
In the CRE area, UE can only camp on the protected cell. For the UE who camps in the protected cell in the CRE area, if it has the capability of CA and CCS, it can take data transfer in both the protected cell and the non-protected cell; otherwise, if it has the capability of CA but not CCS, or it has the capability of neither CA nor CCS, it can take data transfer only in the protected cell.
In the non-CRE area, UE can camp on either the protected cell or the non-protected cell. And UE can take data transfer in both the protected cell and the non-protected cell as long as it has the capability of CA, regardless of CCS.
If the function of CCS is applied to all downlink sub frames of cell 1, the macro cell (cell 1) with the same carrier frequency as the protected small cell lacks of necessary downlink control channels, and hence it cannot accommodate UE for camping. Else, if the function of CCS is applied to a part of downlink sub frames of cell 1, the macro cell (cell 1) with the same carrier frequency as the protected small cell has limited or weakened capability to accommodate UE for camping.
Application of CRE and CCS would result in an area caused by CRE at a certain frequency carrier where the UE is not allowed to access the macro cell or the small cell. The way out for those UEs who are moving toward such area at such frequency carrier is to perform handover to another inter-frequency cell which has been protected by CCS. To handle such scenario, the existing mobility control method is low efficient and hence causes long interruption time and even high drop rate.